1. Field of the Invention
The present invention relates to a color-filterless liquid crystal display (LCD), and more particularly, to a color-filterless LCD having a large screen.
2. Description of the Related Art
In general, liquid crystal displays (LCDs) include a backlight unit which uniformly emits white light and an LCD unit. Since the LCD unit only transmits or blocks the white light generated by the backlight unit, the LCDs also require color filters that respectively transmit red (R), green (G), and blue (B) light beams to produce color images. However, since each of the color filters filter only one of the red (R), green (G), and blue (B) light beams passing through the LCD unit, the LCDs achieve a transmission rate of only 30% or so. Considering light loss through other optical components, only 10% of the light emitted from the backlight unit is actually transmitted to viewers. The light loss of the LCDs mainly occurs in the color filters. Also, the color reproducibility of the LCDs greatly depends on the performance of the color filters.
To address these problems, a color-filterless LCD device which can produce color images without color filters has been developed. FIG. 1 is a cross-sectional view of a conventional color-filterless LCD. Referring to FIG. 1, the conventional color-filterless LCD includes a backlight unit 20 that emits light beams at different exit angles according to wavelengths and an LCD unit 10 that includes optical elements for focusing light beams having predetermined wavelengths on predetermined pixels.
The backlight unit 20 includes a light guide plate (LGP) 21, a plurality of light sources 22 arranged on a lateral side of the LGP 21, and a color separation sheet 23 disposed on a top surface of the transparent LGP 21 and transmitting light beams at different exit angles according to wavelengths. The color separation sheet 23 may be a diffraction grating sheet in which diffraction gratings are periodically arranged in a sinusoidal, prism-like, or quadrangular configuration. The LGP 21 may become thinner away from the light sources 22 to uniformly emit light.
The LCD display unit 10 includes cylindrical lenses 11, a transparent plate 12, a liquid crystal panel 13 having a liquid crystal layer divided into a plurality of pixels, a diffractive optical element (DOE) 14 for transmitting light in a vertical direction, and a transparent plate 15.
In the conventional color-filterless LCD, light incident from the lateral side of the LGP 21 is totally reflected inside the LGP 21, and then is obliquely incident on the top surface of the LGP 21. Part of the light obliquely incident on the top surface of the LGP 21 is totally reflected again, and another part of the light is separated into colored light beams by the color separation sheet 23 and the colored light beams are emitted at different exit angles according to wavelengths from the top surface of the LGP 21. For example, a green (G) light beam is emitted at an exit angle of 0°, a blue (B) light beam is emitted at an exit angle of approximately −10°, and a red (R) light beam is emitted at an exit angle of approximately +10°. Next, the respective light beams are incident on the cylindrical lenses 11. Referring to FIG. 1, each of the cylindrical lenses 11 corresponds to three pixels of the liquid crystal panel 13. The light beams incident on the cylindrical lens 11 converge on different positions according to their incident angles. Referring to FIG. 1, the green (G) light beam converges on the central pixel among the three pixels, the blue (B) light beam converges on the left pixel, and the red (R) light beam converges on the right pixel. Since the light beams are separately incident on the different pixels of the liquid crystal panel 13 according to wavelengths, color images can be produced without color filters.
Since the conventional color-filterless LCD does not need color filters, light loss due to the color filters is prevented. Accordingly, the conventional color-filterless LCD can achieve high brightness. However, the conventional color-filterless LCD has a drawback in that it should only use an edge type backlight unit.
This is because the angle of the light incident on the color separation sheet 23 must be large enough to achieve sufficient color separation by means of the color separation sheet 23. Referring to FIG. 2A, when the light is incident on the color separation sheet 23 at a small incident angle α, the colored light beams are emitted at large exit angles βB, βG, and βR, respectively, such that it is difficult to converge the respective colored light beams on the desired pixels in spite of color separation. To converge the respective colored light beams on the desired pixels, the green (G) light beam may be emitted at an exit angle of 0°, and the red (R) and blue (B) light beams may be emitted at symmetric exit angles. For example, if the incident angle is approximately 70°, the green (G) light beam can be emitted at an angle of approximately 0°, the blue (B) light beam can be emitted at an angle of approximately −10°, and the red (R) light beam can be emitted at an angle of approximately +10°. However, if the conventional color-filterless LCD employs a direct type backlight unit, the LCD fails to achieve sufficient color separation since light is incident on the color separation sheet 23 at a small angle.
The edge type backlight unit can be used only for small LCDs because as the LGP 21 becomes larger, a side opposite to the side where the light sources 22 are installed is darker, thereby degrading brightness uniformity. Also, as the LGP 21 becomes larger, overall brightness is degraded. Accordingly, the edge type backlight unit is applied only to 17-inch or smaller LCDs to ensure optimal brightness and brightness uniformity.